Vacuum is measured as the height of a column of mercury that is a specified number of inches tall. Full vacuum in an industrial process, is considered to be 28” of Hg (mercury); theoretical full vacuum being 30” Hg. Expressed as negative pressure, theoretical full vacuum is the equivalent of one atmosphere (14.7 psi). In a hose application, full vacuum equates to the same 14.7 psi applied externally to the hose. This may seem to be a small amount, but since hose is designed primarily to hold internal pressure, this can be enough to cause collapse under certain conditions and with certain types of hose.